diff --git a/app/vmalert/web.go b/app/vmalert/web.go
index 733faa24f..0b915d9a3 100644
--- a/app/vmalert/web.go
+++ b/app/vmalert/web.go
@@ -197,7 +197,7 @@ func (rh *requestHandler) groups() []APIGroup {
 	rh.m.groupsMu.RLock()
 	defer rh.m.groupsMu.RUnlock()
 
-	var groups []APIGroup
+	groups := make([]APIGroup, 0)
 	for _, g := range rh.m.groups {
 		groups = append(groups, g.toAPI())
 	}
@@ -259,6 +259,7 @@ func (rh *requestHandler) listAlerts() ([]byte, error) {
 	defer rh.m.groupsMu.RUnlock()
 
 	lr := listAlertsResponse{Status: "success"}
+	lr.Data.Alerts = make([]*APIAlert, 0)
 	for _, g := range rh.m.groups {
 		for _, r := range g.Rules {
 			a, ok := r.(*AlertingRule)
diff --git a/app/vmalert/web_test.go b/app/vmalert/web_test.go
index 6271c2a64..bc84f64d9 100644
--- a/app/vmalert/web_test.go
+++ b/app/vmalert/web_test.go
@@ -127,5 +127,59 @@ func TestHandler(t *testing.T) {
 	t.Run("/api/v1/1/0/status", func(t *testing.T) {
 		getResp(ts.URL+"/api/v1/1/0/status", nil, 404)
 	})
-
+}
+
+func TestEmptyResponse(t *testing.T) {
+	rh := &requestHandler{m: &manager{groups: make(map[uint64]*Group)}}
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { rh.handler(w, r) }))
+	defer ts.Close()
+
+	getResp := func(url string, to interface{}, code int) {
+		t.Helper()
+		resp, err := http.Get(url)
+		if err != nil {
+			t.Fatalf("unexpected err %s", err)
+		}
+		if code != resp.StatusCode {
+			t.Errorf("unexpected status code %d want %d", resp.StatusCode, code)
+		}
+		defer func() {
+			if err := resp.Body.Close(); err != nil {
+				t.Errorf("err closing body %s", err)
+			}
+		}()
+		if to != nil {
+			if err = json.NewDecoder(resp.Body).Decode(to); err != nil {
+				t.Errorf("unexpected err %s", err)
+			}
+		}
+	}
+
+	t.Run("/api/v1/alerts", func(t *testing.T) {
+		lr := listAlertsResponse{}
+		getResp(ts.URL+"/api/v1/alerts", &lr, 200)
+		if lr.Data.Alerts == nil {
+			t.Errorf("expected /api/v1/alerts response to have non-nil data")
+		}
+
+		lr = listAlertsResponse{}
+		getResp(ts.URL+"/vmalert/api/v1/alerts", &lr, 200)
+		if lr.Data.Alerts == nil {
+			t.Errorf("expected /api/v1/alerts response to have non-nil data")
+		}
+	})
+
+	t.Run("/api/v1/rules", func(t *testing.T) {
+		lr := listGroupsResponse{}
+		getResp(ts.URL+"/api/v1/rules", &lr, 200)
+		if lr.Data.Groups == nil {
+			t.Errorf("expected /api/v1/rules response to have non-nil data")
+		}
+
+		lr = listGroupsResponse{}
+		getResp(ts.URL+"/vmalert/api/v1/rules", &lr, 200)
+		if lr.Data.Groups == nil {
+			t.Errorf("expected /api/v1/rules response to have non-nil data")
+		}
+	})
 }
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 28a45a5ef..dd6d291b3 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -17,6 +17,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): Properly form path to static assets in WEB UI if `http.pathPrefix` set. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4349).
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): Properly set datasource query params. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4340). Thanks to @gsakun for [the pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/4341).
+* BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): properly return empty slices instead of nil for `/api/v1/rules` and `/api/v1/alerts` API handlers. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4221).
 
 ## [v1.79.13](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.79.13)